Stereophonic sound reproducing system



0ct. 23, l

962 H. E. Dow 3,060,266

STEREOPHONIC SOUND REPRODUCING SYSTEM Filed March 14, 1960 Q/ za Muff/via /f/ /30 33 5f I L fz/zy Z2 2f Z7 P//Aff l Wi/erik 3,@@265 Fatented Oct. 23., 1962 3,060,266 STEREUPHONEC SOUND REPRGDUCING SYSTEM Harrison E. Dow, Wycombe, Pa., assigner, by mesne assignments, to Philco Corporation, Philadelphia, Pa., a corporation of Delaware Filed Mar. 14, 1960, Ser. No. 14,704 12 Claims. (Cl. 179-1) This invention relates to sound reproduction and more particularly to stereophonic sound reproduction.

Among the developments in stereophonic sound reproduction is the so-called common lows type of system in which all of the low frequency signal components in both the left and the right channels are reproduced by a single sound reproducer or loud speaker. This type of system is based on the fact that the human ears have little, if any, sense of directional hearing for the low frequency signal components below a frequency of about 300 cycles. While a number of forms of this type of system have been proposed, they have generally required at least one output transformer-sound reproducer cornbination constructed to reproduce substantially the entire audio spectrum adequately. A full-ranUe transformer is usually a relatively expensive device which may involve special core materials and special windings. Similarly a full-range loud speaker is also known to be expensive to produce.

One object of the present invention is to provide an improved stereophonic sound reproducing system of the common lows type.

Another object of the invention is to provide a system which does not require any full-range transformers or sound reproducers.

Another object of the invention is to provide a system in which faithful reproduction of stereophonic sound may be achieved with relatively inexpensive devices which are required to handle only restricted ranges of the audio signals.

A further object of the invention is to provide a common lows system in which relatively inexpensive output transformers may be utilized for the reproductio-n of the higher frequency components of the audio signals, and in which the output transformer for the low frequency components may also be constructed at relatively low cost.

Still another object o-f the invention is to provide a system which utilizes the push-pull principle to the fullest extent in order to achieve the desired results.

Other objects and features of the invention will be apparent as the description proceeds.

In accordance with this invention, there is provided a stereophonic sound-reproducing system wherein pushpull output stages are employed, each including a transformer which may `be constructed to transfer substantially only those signal components whose frequencies are above a predetermined frequency, and there is also employed a transformer which is common to said stages and which preferably is caused to transfer substantially only those signal components whose frequencies are below said predetermined frequency. A sound reproducer is connected to the secondary of each transformer, and each reproducer preferably is constructed to reproduce substantially only the signal components which are freely transferred by the associated transformer. The arrangement according to the invention is such that all of the transformers are connected for push-pull operation, and the result is that the advantage of push-pull operation is imparted to both the stereophonically reproduced highs and to the monophonically reproduced lows, and this enables the use of inexpensive transformers.

The invention may be fully understood from the following detailed description with reference to the accompanying drawing wherein:

FIG. l is a schematic diagram of a stereophonic sound reproducing system embodying a preferred form of the present invention; and

FIG. 2 is a similar diagram of a stereophonic sound reproducing system embodying another form of the-invention.

Referring first to FIG. l, the blocks 10 and 11 are intended to represent any two sources of stereophonic signals such as the output of a stereophonic phonograph cartridge, or the output of FM-AM stereophonic channels, or the output of FM-FM multiplex stereophonic channels. A rst push-pull amplifier stage 12 is connected to receive the signal from source 10. The said stage is shown as employing vacuum tubes 13 and 14, although of course transistors could be used. The cathodes of said tubes are tied together and share a common parallel resistance-capacitance path to ground. The signal from source 1li is supplied directly to the control grid of tube 13, and is supplied through a phase inverter 1S to the control grid of tube 14. The phase inverter 15 may be of any known form, for example it may be a simple Ivacuum tube inverter. The `output transformer 16 preferably is constructed to transfer substantially only those signal components whose frequencies are above a predetermined frequency, eg. 300 cycles, and it comprises two primary sections, i.e. windings 17 and 18, and a secondary winding 19. That is to say, transformer 16 preferably is constructed to have a low cut-olf at a predetermined frequency. A sound reproducer 2@ is connected to the secondaryv winding 19, and it preferably is constructed to reproduce substantially only those signal components whose frequencies are above said predetermined frequency, since it is only required to reproduce those signal components.

A second push-pull amplifier stage 21, similar to stage 12, is connected to receive the signal from source 11. It too is shown `as comprising vacuum tubes 22 and 23, phase inverter 24, and output transformer 25 which preferably is also constructed to transfer substantially only those signal components whose frequencies are above the aforementioned predetermined frequency. Transformer 25 comprises two primary sections, i.e. windings 26 and 27, and a secondary winding 23. A sound reproducer 29 is connected to said secondary winding, and it preferably is constructed to reproduce substantiallyonly the signal components whose frequencies are above said predetermined frequency.

Common to the two push-pull stages is a third transformer 39 which is intended to transfer primarily signal components whose frequencies are below said predetermined frequency. in the form shown it comprises two primary sections provided by the center-tapped primaryv winding 31, and a secondary winding 32. The signal components transferred by the transformer 30 are supplied to a sound reproducer 33 which preferably is constructed to reproduce substantially only the said lower frequency components since it is only required to reproduce those components. In fact, this sound reproducer need only be a woofer and not a reproducer or loud speaker having substantial range.

Of particular interest is the arrangement whereby transformer 30 is common to the two push-pull stages 12 and 21. A common B-lsupply is connected to the center-tap of primary winding 31. The lower end of winding 17 and the upper end of winding 1S are connected respectively to the upper and lower ends of winding 31. Similarly, the lower end of winding 26 and the upper end of winding 27 are connected respectively to the upper and lower ends of winding 31. It will be seen therefore that the primary` and are connected in push-pull relation through the primary winding 31 of transformer 30. It will also be seen that the sections of winding 31 defined by the center tap are connected to push-pull relation in the outputs of both of the push-pull stages. The upper section of winding 31 is included in the plate circuit of tube 13 in series with winding 17, while the lower section of winding 31 is included in the plate circuit of tube 14 in series with winding 18. Similarly, the upper section of Winding 31 is included in the plate circuit of tube 22 in series with winding 26, while the lower section of winding 31 is included in the plate circuit of tube 23 in series with wind- .ing 27. A capacitor 34 is connected across the primary winding 31, and it has a capacitance value such that it presents negligible impedance to signals whose frequencies are above the aforementioned predetermined frequency but relativelyhigh impedance to signals whose frequencies are below said predetermined frequency.

In operation the transformers 16 and 25 freely transfer the higher frequency signal components to sound reprodueers 20 and 29 but preferably do not have any appreciable response to the low frequency components. Therefore the low frequency signal currents pass through the primary windings of said transformers without inducing any appreciable voltage in their secondaries. However, these currents, passing through the primary winding 31 of transformer 30, induce signal voltage of very substantial magnitude in secondary winding 32, especially in view of the push-pull action. Transformer does not transfer any appreciable higher frequency currents, since such currents are by-passed by the capacitor 34. Of course, two capacitors in series with one another with their junction grounded, or connected to B+, could be used instead of the single capacitor shown, and such arrangement would be even more effective. However, the extent of the resulting improvement may not warrant the additional cost.

From the foregoing description, it will be seen that the higher frequency signal components are stereophonically reproduced while the low frequency signal components are monophonically reproduced. Since, as previously stated, the human ears have little, if any, sense of directional hearing at the low frequencies below about 300 cycles, the system effectively gives faithful stereophonic reproduction.

It will also be seen that this invention utilizes the pushpull principle to the fullest possible extent. Since both output stages are push-pull stages and all three output transformers are constructed and connected for push-pull operation, the D.C. current in the two sections of each primary has no adverse effect. Consequently, it is possible to employ inexpensive transformers. Of course, this is also made possible by the fact that the transformers are only required to transfer signal components in restricted frequency ranges. This invention also makes it possible to reduce the cost of the sound reproducers which are only required to reproduce the signal components in the restricted ranges. Thus this invention enables substantial cost reduction in stereophonic reproducing systems.

Referring now to FIG. 2, there is shown another embodiment of the invention. Since all of the components except transformer 30a are the same as the corresponding components in FIG. l, they are designated by the same reference numerals. In this instance, the transformer 30o, corresponding to transformer 30 of FIG. 1, has two centertapped primary windings 31a and 311; providing four primary sections, and the B-lsupply is connected to both center taps. I'he two primary windings 31a and 31b are connected respectively to the primary windings of transformers 16 and 25 in the same manner that the primary winding 31 in FIG. l is connected to the primary windings of said transformers. Thus the primary windings 31a and 31b are included respectively in the outputs of the two connected in push-pull relation in the respective outputs of the two push-pull stages. However, transformer 30a as a Whole is common to the two stages and transfers the low frequency signal components from both stages to sound reproducer 33. The operation is the same as in the system of FIG. l, the only difference being that the low frequency signal currents from the two channels are transferred via different primary windings. For the reason that transformer 30a is more expensive to construct than transformer 30, the system of FIG. 2 is less desirable than the system of FIG. 1.

While a system according to this invention, such as shown in FIGS. l and 2, obviates the necessity of employing full-range transformers and loud speakers, such transformers and loud speakers could be employed. The low frequency sounds would then emanate from all three speakers with a corresponding reduction in the bass from speaker 33.

While certain embodiments of the invention have been illustrated and described, it will be understood that the invention is not limited thereto but contemplates such modifications and other embodiments as may occur to those skilled in the art.

I claim:

1. In a stereophonic sound reproducing system, a first push-pull output stage to which one stereophonic signal is supplied, said stage including ya rst output transformer having a two-section primary winding and a secondary winding, the two primary winding sections being connected for push-pull operation, a irst sound reproducer connected to said secondary winding, a second push-pull output stage to which another stereophonic signal is supplied, said second stage including a second output transformer also having a two-section primary winding and a secondary winding, the two primary winding sections of said second transformer being connected for push-pull operation, a second sound reproducer connected to said last-mentioned secondary winding, a third output transformer having a plurality of primary winding sections and a secondary winding, means connecting two primary winding secti-ons of said third transformer respectively in series with the two primary winding sections of said first transformer, means connecting two primary winding sections of said third transformer respectively in series with vthe two primary winding sections of said second transformer, and a third sound reproducer connected to the secondary winding of said third transformer.

2. A stereophonic sound reproducing system according to claim l, wherein said first and second transformers are characterized in that they transfer substantially only those signal components whose frequencies are above a predetermined frequency, and said system also includes means for causing said third transformer to transfer substantially only those signal components whose frequencies are below said predetermined frequency.

3. A stereophonic sound reproducing system according to claim 2, wherein said first and second sound reproducers are characterized in that they reproduce substantially only those signal components whose frequencies are above said predetermined frequency, and said third sound reproducer is characterized in that it reproduces substantially only those signal components whose frequencies are below said predetermined frequency.

4. A stereophonic sound reproducing system according to claim l, wherein said third transformer has only two primary winding sections.

5. A stereophonic sound reproducing system according to claim l, wherein said third transformer has four primary winding sections, two connected respectively in series with the two primary winding sections of said rst transformer and the other two connected respectively in series with the two primary winding sections of said second transformer.

6. In a stereophonic sound reproducing system, a rst push-pull stages, and the sections of said windings are push-pull output stage to which one stereophonic signal is supplied, said stage including a first pair of amplifier devices each having input and output electrodes and a first output transformer characterized in that it transfers substantially only those signal components whose frequencies are above a predetermined frequency, said transformer having a two-section primary winding and a secondary Winding, a first sound reproducer connected to said secondary winding to receive said signal components, a second push-pull output stage to which another stereophonic signal is supplied, said second stage including a second pair of amplifier devices each having input aud output electrodes and a second output transformer constructed to transfer substantially only those primarily signal components whose frequencies are -above said predetermined frequency, said second transformer also having a two-section primary winding and a secondary winding, a second sound reproducer connected to said last-mentioned secondary winding to receive the signal components transferred by said second transformer, a third output transformer having a plurality of primary winding sections and a secondary winding, a first push-'pull output circuit connected to the output electrodes of said first pair of devices and including in the respective push-pull branches the two primary winding sections of said first transformer and also including in the respective branches two primary winding sections of said third transformer, a second push-pull output circuit connected to the output electrodes of said Second pair of devices and including in the respective pushpull branches the two primary winding sections of said second transformer and also including in the respective branches two primary winding sections of said third transformer, lmeans for causing said third transformer to transfer substantially only those signal components whose frequencies are below said predetermined frequency, and a third sound reproducer connected to the secondary ywinding of said third transformer to receive the signal components transferred by said third transformer.

7. A stereophonic sound reproducing system according to claim 6, wherein said first and second sound reproducers are characterized in that they reproduce only signal components whose frequencies are above said predetermined frequency, and said third sound reproducer is characterized in that it reproduces only signal components Whose frequencies are below said predetermined frequency.

8. A stereophonic sound reproducer according -to claim 6, wherein said third transformer has only two primary winding sections which are included respectively in the push-pull branches of each of said push-pull output circuits.

9. A stereophonic sound reproducer according to claim 6, wherein said third transformer has four primary winding sections, two included respectively in the push-pull branches of said first push-pull output circuit, and the other two included respectively in the push-pull branches of said second push-pull output circuit.

10. In a stereophonic sound reproducing system, a first push-pull output stage to which one stereophonic signal is supplied, said stage including a first pair of amplifier devices each having input and output electrodes and a first output transformer characterized in that it transfers substantially only those signal components Whose frequencies are above a predetermined frequency, said transformer having a two-section primary winding and a secondary Winding, means connecting the primary winding sections to the output electrodes of said pair of devices for push-pull operation, a sound reproducer connected to said secondary winding to receive said signal components, a second push-pull output stage to which another stereophonic signal is supplied, said second stage including a second pair of amplifier devices each having input and output electrodes and a second output transformer constructed to transfer substantially only those signal components Whose frequencies are above said predetermined frequency, said second transformer also having a twosection primary Winding and a secondary Winding, means connecting the primary winding sections of said second transformer to the output electrodes of said second pair of devices for push-pull operation, a second sound reproducer connected to said last-mentioned secondary Winding to receive the signal components transferred by said second transformer, a third output transformer having a center-tapped primary winding and a secondary winding, a source of operating voltage connected to the center tap of said primary Winding, means connecting the primary Winding sections of said first transformer respectively to the ends of said primary winding so as to provide a `first push-pull output circuit each of Whose branches includes a primary winding section of said first transformer and a primary Winding section of said third transformer, means connecting the primary winding sections of said second transformer respectively to the ends of said primary Winding so as to provide a second pushpull output circuit each of Whose branches includes a primary winding section of said second transformer and a primary winding section of said third transformer, means for causing said third transformer to transfer substantially only those signal components Whose frequencies are below said predetermined frequency, and a third sound reproducer connected to the secondary winding of said third transformer to receive the signal components transferred by said third transformer.

ll. In a stereophonic sound reproducing system, a first push-pull output stage to Which one stereophonic signal is supplied, said stage including a first pair of amplifier devices each having input and output electrodes and a rst output transformer characterized in that it transfers primarily signal components Whose frequencies are above a predetermined frequency, said transformer having a two-section primary winding and a secondary Winding, means connecting the primary Winding sections to the output electrodes of said pair of devices for push-pull operation, a first sound reproducer connected to said secondary winding to receive said signal components, a second push-pull output stage to which another stereophonic signal is supplied, said second stage including a second pair of amplifier devices each having input and output electrodes and a second output transformer characterized in that it transfers primarily signal components whose frequencies are above said predetermined frequency, said second transformer also having a two-section primary Winding and a secondary winding, means connecting the primary winding sections of said second transformer to the output electrodes of said second pair of devices for push-pull operation, a second sound reproducer connected to said last-mentioned secondary winding to receive the signal components transferred by said second transformer, a third output transformer having two center-tapped primary windings and a secondary Winding, a source of operating voltage connected to the center taps of said primary windings, means connecting the primary winding sections of said first transformer respectively to the ends of one of said primary windings so as to provide a first push-pull circuit each of whose branches includes a primary Winding section of said first transformer and a primary winding section of said third transformer, means connecting the primary winding sections of said second transformer respectively to the ends of the other of said primary windings so as to provide a second push-pull circuit each of Whose branches includes a primary winding section of said second transformer and a primary Winding section of said third transformer, means for causing said third transformer to transfer substantially only those signal components whose frequencies are below said predetermined frequency, and a third sound reproducer connected to the secondary winding of said third transformer to receive the signal components transferred by said third transformer.

12. In a stereophonic system, a first push-pull stage to which one stereophonic signal is supplied, said stage including a first output transformer having a two-section 7 primary Winding and a secondary winding, the two primary winding sections being connected for push-pull operation, a second push-pull stage to which another stereophonic signal is supplied, said second stage including a second output transformer also having a two-section primary winding and a secondary winding, the two primary Winding sections of said second transformer being connected for push-pull operation, a third output transformer having a plurality of primary winding sections and a secondary winding,

w means connecting two primary winding sections of said third transformer respectively in series with the two primary Winding sections of said rst transformer, and means connecting two primary Winding sections of said third transformer respectively in series with the two primary winding sections of said second transformer.

No references cited. 

